1. Field of the Invention
The present invention relates to a fixing device which fixes an unfixed toner image on a recording material as a permanent image by heating/pressing the recording material having the unfixed toner image.
2. Description of the Related Art
A typical conventional image fixing device used in an image forming apparatus, in which a toner image is transferred onto a recording material as an unfixed image, and a permanent image is provided by fixing the unfixed image by heating/pressing the recording material, is configured as shown in FIG. 1. In FIG. 1, a fixing roller 1 is a fixing rotating member, serving as recording-material conveying means, and includes a releasing layer 12 made of a fluororesin, such as PTFE (polytetrafluoroethylene) or the like, formed on a core bar 11 made of aluminum, iron or the like, and a heater 3, serving as heating means, incorporated within the fixing roller 1 for heating the fixing roller 1 from inside.
A pressing roller 2 is a pressing rotating member, and is pressed against the fixing roller 1 by pressing means (not shown) to form a fixing nip N. The pressing roller 2 includes a releasing layer 23 made of a fluororesin, such as PFA, PTFE or the like, having an excellent releasing property, formed on an elastic layer 22 made of a heat-resistant silicone sponge rubber or the like, which is formed on a core bar 21 made of aluminum, iron or the like.
The releasing layer 12 prevents an "offset" phenomenon in which a toner image is transferred onto the fixing roller 1 to stain the succeeding image. The offset phenomenon is more assuredly prevented by applying a bias voltage having the same polarity as the polarity of toner to the core bar 11 from a fixing bias power supply 8 by utilizing static charges particular to the toner. (While FIG. 1 illustrates a case in which the toner has a negative polarity, a positive fixing bias polarity is applied to the toner when the toner has a positive polarity.)
The releasing layer 23 prevents the offset toner transferred from the recording material toward the pressing roller 2 via the fixing roller 1 and the toner adhering to the back surface of the recording material from adhering to the surface of the pressing roller 2.
The cathode and the anode of a diode 9 are connected to the core bar 21 of the pressing roller 2 and the ground of the main body of the apparatus, respectively, in order to prevent the potential of the core bar 21 of the pressing roller 2 from being negative. Thus, a potential difference is provided between the surface of the fixing roller 1 and the core bar 21 of the pressing roller 2, to prevent the offset phenomenon by the electric field produced by the potential.
Unfixed toner 7 is fixed on the recording material by being heated/pressed at the fixing nip N.
An entrance guide 4 guides the recording material 6 having the unfixed toner image formed thereon so as to be conveyed to the fixing nip N, where the unfixed toner image is fixed as a permanent image on the recording material 6.
A thermistor 5, serving as a temperature detection sensor for detecting the surface temperature of the fixing roller 1, contacts the surface of the fixing roller 1 with a predetermined contact pressure, and turns on/off (an electric circuit for turning on/off power supply is not shown) power supply to the heater 3 so that the temperature of the fixing roller 1 is maintained constant.
When forming the releasing layer 12 of the fixing roller 1 with a PFA (a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) resin, although thermoplasticity is superior to other fluororesins, a current flows through a local portion when a bias voltage is applied, so that the releasing layer 12 is degraded at that portion, to degrade the releasing property and to cause adherence of toner. A portion where the toner adheres causes a partial defect in the fixed toner image formed on the recording material. As an effective method for solving this problem, the use of a PFA resin having end groups of --CF.sub.3 has been proposed (Japanese Patent Laid-Open Application (Kokai) No. 5-72934 (1993)).
When forming the releasing layer 12 of the fixing roller 1 with the PFA resin having end groups of --CF.sub.3, although it is advantageous to also form the releasing layer 23 of the pressing roller 2 with a PFA resin from the viewpoint of releasability, there is a problem of causing an electrostatic offset phenomenon in toner.